Coxiella burnetii is an obligate intracellular bacterium and the causative agent of the zoonosis human Q (query) fever. Acute Q fever normally manifests as a self-limiting influenza-like illness. Rare but serious chronic infections can occur that usually presents as endocarditis or hepatitis. Coxiella exhibits considerable strain diversity. Strains can be grouped according to an association with human acute or chronic disease, suggesting that groups have unique virulence potential. The vast majority of human Q fever cases are acquired though contact with infected domestic livestock where the organism can be endemic. C. burnetii can chronically infect a variety of animals and is shed in large numbers in various secretions and products of parturition. Adding to the insidious nature of the organism is an infective dose approaching one organism and a remarkable extracellular stability approaching that of a bacterial spore. Environmental resistance also correlates with resistance to the degradative conditions of a phagolysosome-like parasitophorous vacuole (PV), Coxiella's niche within host macrophages. The impressive environmental stability of C. burnetii is likely due to the biogenesis of a highly resistant cell form termed the small cell variant (SCV). This form arises during a biphasic developmental cycle and is likely responsible for the majority of environmentally acquired cases of Q fever. Once internalized and sequestered in a PV, SCV morphologically differentiate into more metabolically and replicatively active large cell variants (LCV). Mature PV contain a mixture of SCV, LCV and intermediate forms. The molecular biology of C. burnetii morphological differentiation is poorly understood. Important areas of future investigation include identification of the cellular conditions and signal transduction that drive development, the kinetics of development, the relative infectivity of SCV and LCV for various hosts, the transcriptional and translational capabilities of cell forms, and the biochemical composition of SCV and LCV that confer their unique biological properties. In vitro C. burnetii replicates within an array of epithelial, fibroblast and macrophage-like cell lines. In vivo the initial target is the alveolar macrophage although the organism can subsequently disseminate to replicate within a wide variety of tissues. In some cases C. burnetii establishes a persistent infection that can reactivate months or years after initial exposure resulting in serious disease such as endocarditis. The mechanisms by which Coxiella evades clearance by the host immune response during persistent infection are unknown but likely involve both pathogen and host factors. Phagocytic dendritic cells (DC) are exceptional antigen presenting cells that bridge the innate and adaptive immune responses. The role of DC in controlling replication of C. burnetii is unknown. Moreover, the growth characteristics of isolates of differing virulence potential in both DC and primary human macrophages is poorly defined. C. burnetii is unique among intracellular bacteria in residing within a large and spacious lysosome-like PV. The cellular signaling and vesicular trafficking pathways necessary for biogenesis of this vacuole are unknown. Furthermore, the biochemical nature of the PV lumen and membrane are unknown. In all cases examined, invasive bacteria modify their PV to enhance survival and usually growth. Bacterial effectors of PV remodeling generally interact with host molecules that regulate vesicular trafficking. The nature and extent of modification of the C. burnetii PV has not been defined. Coxiella contains a near compete copy of the type IV secretion apparatus genes of Legionella pneumophila, a phylogenetically close relative. Because Legionella requires functional type IV secretion to establish its replicative niche, it is logical to assume a similar requirement for Coxiella.